I'm not sure if this is done the same way in all calculators. I have a Casio fx-991ES and here's how I convert from base-10 to base-2:

1. First, switch your current mode to Base-n mode
2. Make sure that you're set to base-10 (dec).
3. Input your numbers.
4. Press any button that has an indication of the word binary (bin, base-2, etc..).